Asymmetric membranes formed of various polymeric materials have been conventionally known. Examples of such asymmetric membranes include membranes using fluorine-containing polyimides as coating materials (e.g., JP 05-7749 A and JP 06-188167 A), a membrane using a polyacrylonitrile (e.g., JP 05-184891 A), a membrane using a polyolefin (e.g., JP 2002-535115 A), a membrane using a polyethersulfone (e.g., JP 09-285723 A), and membranes using poly(1-trimethylsilyl-1-propyne) (PTMSP) (e.g., JP60-1323605 A and JP 02-222715 A). All of these membranes are intended for gas separation or liquid-liquid separation; they do not contain a filler, nor do they provide sufficient gas permeability.
Membranes prepared by sol-gel reactions intended for liquid-liquid separation are known as separation membranes containing fillers (e.g., JP 11-192420 A and JP 11-9976 A). These membranes, however, are not also suitable as membranes intended for gas permeation.
The recent technological advances have made it possible to increase air-tightness even in spaces such as automobiles, where increasing the air-tightness has been difficult. When many passengers are aboard an automobile with such high air-tightness for long hours, a decrease in oxygen concentration or an increase in carbon dioxide concentration will occur, which may cause the passengers a headache or an unpleasant feel. Thus, the introduction of an adequate amount of outside air is necessary.
However, urban roads, highways, and the like are contaminated with pollutants such as dust; therefore, direct introduction of outside air into automobiles has been a big problem in view of the passengers' health. One example of a method for solving this problem is a method in which an inlet for introducing outside air is provided with a filter for removing pollutants in the air such as suspended matter.
Non-woven cloths, mechanical filters, and the like have been conventionally used as such filters. In JP 2004-203367 A, an air-conditioning system for the entire automobile is suggested.
However, conventional filters such as non-woven cloths and mechanical filters had a problem in that they cannot sufficiently remove suspended matter in the air having particle sizes of 10 μm or less (hereinafter referred to as “SPM”). Among SPM, particularly nanoparticles having particle sizes on the order of 10 nanometers, when inhaled into a human body, are considered to reach the lower respiratory tract such as the bronchium and alveo, and be easily deposited. Thus, there is a strong desire for the development of an air-conditioning system that can sufficiently block these nanoparticles.
The use of a gas-permselective membrane made of a polymeric material as a filter enables removal of SPM to some extent; in this case, however, the gas permeability is insufficient, and the object of introducing sufficient outside air cannot be achieved.